From your responses, it seems that identifying the need should be the first step. We should involve other departments, teach them about Web 2.0 technologies and how it can benefit them, and get their input of what their business needs are.
Secondly we (IT) should implement a policy, for compliance, requirements, support and maintenance.
Third, build it.
Fourth, train the users and maintain it.
Who should be responsble for its content such as moderators, individual departments for their own content, webmasters or IT?